The objectives are: 1) to determine the role of vitamin D in adaptation of calcium metabolism in the lactating rat with particular regard to interactions with parathyroid hormone (PTH) and calcitonin; and 2) to determine the effects of vitamin D status of the suckling pups on calcium and bone metabolism and secretion of calcitonin and parathyroid hormone. The role of vitamin D in the lactating rat will be determined: a) from the sequential changes in the serum levels of calcium, 1, 25-(OH)2D, parathyroid hormone and calcitonin in early lactation; b) from the effects of injections of 1,25-(OH)2D3 on the serum levels of calcium and phosphate and on intestinal calcium absorption (in vivo and in vitro) in vitamin D-deprived, hypocalcemic lactating rats; c) from the effects of consumption of diets with different vitamin D contents on the serum levels of calcium, phosphate and 25-OHD3 in lactating rats and their pups; and d) from the effects of dietary vitamin D status on the rate of secretion of calcitonin and parathyroid hormone in vivo and from thyro-parathyroid glands incubated in vitro. Deprivation of the pregnant and lactating mother of vitamin D will be used to achieve vitamin D deficiency in the suckling pups as a means of determining the earliest effects of such defciency on: a) serum levels of calcium, phosphate, magnesium, 25-OHD3 and alkaline phosphatase; b) weight of bone and bone ash; and c) oxygen uptake, lactic acid release, alkaline phosphatase release from tibia maintained in culture. Reversal of the effects of vitamin D deficiency will be investigated by adding various vitamin D metabolites to the bone culture medium. Hypervitaminosis D as well as hypovitaminosis D in suckling pups will be investigated with regard to: a) the activity of acid phosphatases in bone; b) the role of calcitonin in protection against hypercalcemia; and c) serum levels of PTH. Control of secretion of PTH will be further studied by determining the effect of vitamin D metabolites added in vitro and on the rate of release of PTH from thyroparathyroid glands.